Toner concentration monitoring apparatus

ABSTRACT

Apparatus for continuously monitoring the concentration of toner in an electrographic developer mixture carried on a development mechanism by sensing the reflectivity of the mixture. A source of radiant energy, periodically energized at a selected frequency, is directed at the developer mixture and the reflectance thereof is monitored by a photoelectric transducer which produces a first output signal representative of the intensity of such reflectance. A second photoelectric transducer illuminated directly by the source produces a second output signal representative of the intensity of the radiation emanating from the source as modulated by the surrounding environment. The first and second output signals of the two transducers are coupled to high pass filters which eliminate signals generated by the transducers representative of ambient light and electrical noise. The a.c. output signals from the filters are converted to d.c. signals corresponding to the first and second output signals of the transducers, which d.c. signals are used to control the concentration of toner in the mixture in two alternative ways. In one embodiment, the d.c. signals are fed to a mechanism which produces a control signal proportional to the ratio of their amplitudes, which ratio is proportional to the proportion of the mixed materials. In a second embodiment, a toner replenisher is activated in response to a predetermined change in amplitude of the first d.c. signal. The second d.c. signal is used in a feedback loop for stabilizing the output of the radiation source.

United States Patent 1191 Benwood et al.

[ TONER CONCENTRATION MONITORING APPARATUS [75] Inventors: Bruce R.Benwood, Spencerport;

Theodore H. Morse; Howard D. Siebenrock, both of Rochester, all of N.Y.

[73] Assignee: Eastman Kodak Company,

Rochester, NY.

221 Filed: Aug. 1,1973.

21 App1.No.:384,670

Related U.S. Application Data [63] Continuation-impart of Ser. No.207,226, Dec. 13,

1971', abandoned.

[52] US. Cl 222/57, 222/DIG. 1, 118/637, 250/205 [51] Int. Cl B67d 5/08[58] Field of Search 222/52, 57, 59, 76, DIG. 1; 118/637; 250/205, 211,212, 217 R, 219 SS,

[451 Aug. 20, 1974 [5 7 ABSTRACT Apparatus for continuously monitoringthe concentration of toner in an electrographic developer mixturecarried on a developmcnt'mechanism by sensing the reflectivity of themixture. A source of radiant energy, periodically energized at aselected frequency, is directed at the developer mixture and thereflectance thereof is monitored by a photoelectric transducer whichproduces a first output signal representative of the intensity of suchreflectance.

A second photoelectric transducer illuminated directly by the sourceproduces a second output signal representative of the intensity of theradiation emanating from the source as modulated by the surroundingenvironment. The first and second output signals of the two transducersare coupled to high pass filters which eliminate signals generated bythe transducers representative of ambient light and electrical noise.The ac. output signals from the filters are converted to d.c. signalscorresponding to the first 220 C, 574 and second output signals of thetransducers, which d.c. signals are used to control the concentration ofReferences Cited toner in the mixture in two alternative ways. In oneUNITED STATES PATENTS embodiment, the dc. signals are fed to a mechanism3,053,985 9/1962 Grammer et al 2501212 which Produces a Control SignalProportional to the 3,233,781 2/1966 Grubbs 222/57 ratio of theiramplitudes, which ratio is Proportional to 3,413,480 11/1968 Biard etal.250/211 the proportion of the mixed materials. In a second 3,495,0892/1970 Brown 250/226 X embodiment, a toner replenisher is activated in3.610.205 10/1971 y 222/57 X response to a predetermined change inamplitude of 3,736,431 5/1973 Childs 250/205 h fi d SignaL h secondSignal is used in a feedback loop for stabilizing the output of thePrimary Examiner-Robert B. Reeves radiation Source AssistantExaminer-Joseph J. Rolla Attorney, Agent, or FirmDouglas 1. Hague 11Clalms, 5 Drawlng Flgul'es l H W [1 H5 ran 15/ TONE; I? M. DETECTORREPLE/V/SHER PAIENTEumszoasu -suzmors k f l 3 3$ 953% 95? SM Nu PATENIEQms 2 0 1914 FIG. 2

DETECTOR REPLE/V/SHER F/LTER GENERATOR PEAK DETECTOR TONER CONCENTRATIONMONITORING APPARATUS CROSS-REFERENCE TO RELATED APPLICATIONS Referenceis made to commonly assigned US Pat. application Ser. No. 207,226, nowabandoned, entitled TONER CONCENTRATION MONITORING APPA- RATUS, filed onDec. 13, 1971, in the names of Bruce R. Benwood, Theodore H. Morse andHoward D. Siebenrock of which the present application is a continuationin part.

BACKGROUND OF THE INVENTION The present application is acontinuation-in-part of US. Pat. Ser. No. 207,226, now abandon, filed onDec. 13, 1971 in the names of Bruce R. Benwood, Theodore H. Morse andHoward D. Siebenrock.

This invention relates to electrographic development apparatus forcontrolling the concentration of electroscopic toner particles in anelectrographic developer. More specifically, this invention relates toimprovements in toner concentration monitoring apparatus of the typewhich sense toner concentration by sensing variations in reflectivity ofthe developer.

In the electrographic reproduction process, the surface of aradiation-sensitive plate, generally comprising a layer ofphotoconductive material disposed on a conductive backing, is given auniform electrostatic charge and is then imagewise exposed to a patternof actinic radiation corresponding to the indicia on a document or thelike being reproduced. Such exposure serves to selectively dissipate theuniform charge on the surface, leaving behind a latent electrostaticimage which can then be developed by contacting it with anelectrographic developer.

In general, electrographic developers comprise a mixture of suitablypigmented or dyed resin-based electroscopic particles, known as toner,and a granular carrier material which functions to carry such toner bygenerating triboelectric charges thereon. As mentioned above,development of the latent electrostatic image occurs when the developermixture is brought into contact with the electrostatic image-bearingsurface. Such contact is commonly effected by either cascading themixture over such surface or, as is becoming increasingly prevalent,subjecting the surface to the periphery of one or more rotating magneticdevelopment brushes, the bristles of which comprise chain-like arrays oftoner-coated carrier particles. Upon contacting the electrostaticimage-bearing surface, the toner particles, being charged to a polarityopposite to that of the electrostatic image, are separated from thecarrier particles and are selectively deposited on the surface to form adeveloped or toner image which may thereafter be transferred to a paperreceiving sheet and fixed thereto by any suitable means, such as heat,to form a copy of the original document.

Obviously, as toner images are repetitively formed, toner particles arecontinuously depleted from the developing mixture, requiring subsequentreplenishment to avoid a gradual reduction in image density.

To avoid the necessity of manual replenishment and the operatingdifficulties often encountered as a result of overreplenishment, avariety of devices has been heretofore proposed for automaticallyreplenishing toner particles after a predetermined number of copies aremade or, alternatively, after the concentration of toner particles inthe developer mixture drops below a predetermined level. One suchdevice, disclosed in US. Pat. No. 3,233,781, issued to W. J. Grubbs,utilizes the difference in reflectivity exhibited by toner and carrierparticles as a means for monitoring the concentration of toner particlesin the developer mixture. Toner particles, usually being black andpossessing highly absorbing surfaces, reflect less radiant energy thanthe carrier particles. Thus, the reflectivity of the developer mixturedepends upon the relative proportions of the mixed particles, the higherthe concentration of carrier particles, the higher the reflectivity ofthe mixture, and vice versa. According to the Grubbs disclosure, thereflectance of the developer mixture is monitoring by directing energyemanating from an incandescent lamp toward the mixture and detecting theenergy reflected by the mixture with a photoconductive cell. Suchphotocell, together with a similar photocell which is illuminateddirectly by. the lamp and thereby acts as a reference signal, isemployed as a variable resistance arm of a bridge circuit which iscapable of activating a toner replenishing device in response to apredetermined change in the ratio of photocell outputs, such changebeing characterized by an unbalance in the circuit.

While operable, photoelectric toner monitoring devices of the typedescribed above have not proven entirely satisfactory in operation,especially over extended periods of time. A principal cause ofunsatisfactory performance is the airborne toner particles circulatingin the environment in which the monitoring apparatus is employed. Theseairborne toner particles accumulate on the external surface of thevarious components of the monitoring apparatus, including the lamp,thereby gradually reducing the quantity of light received by thedetecting photocell. The low level of received energy causes thephotocell to produce a false control signal indicating a greaterconcentration of toner than is actually present in the mixture. Toassure freedom from erroneous measurements of this type, such monitoringapparatus requires frequent cleaning maintenance.

Other causes of difficulty can be attributed, at least in part, to theincandescent lamp used for illuminating the development mixture, aluminous energy source of relatively low output, unstable intensity andshort life, and the fact that spectral characteristics of the lamp areclosely akin to the background light, thereby making backgrounddiscrimination difficult. The reference photocell and bridge circuit ofthe Grubbs apparatus provide a means to compensate for fluctuations inthe intensity of the output of the lamp itself. The reference photocelland bridge circuit cannot, however, compensate for the graduallydecreasing output detection caused by toner particles accumulating onits exterior surface as discussed in the preceding paragraph.

Detection of ambient light is partially controlled in the Grubbsapparatus by enclosing the reference photocell and lamp within ahousing. The problem is not eliminated, however, because the detectingphotocell is not similarly enclosed and will, therefore, sense ambientlight present in the development mechanism.

Another source of uncompensated error in the Grubbs apparatus is therandom electrical perturbations, commonly called noise, which arereceived and generated by both the detecting and reference photocells.

SUMMARY OF THE INVENTION It is, therefore, an object of the presentinvention to improve photoelectric devices adapted for monitoring andmaintaining substantially constant the relative proportion of mixedmaterials having divergent reflectance characteristics.

Another object of the invention is to provide an optical electrographictoner monitoring apparatus which is insensitive to ambient environmentalconditions.

These and other objects of the invention are achieved by the provisionof a substantially improved apparatus for monitoring, during theoperation of an electrographic copier, the reflectivity of the developerused therein.

Developer reflectance monitoring is accomplished by directing radiationemanating from a source, periodically energized at a selected frequency,at a portion of the developer wherein the toner concentration ischaracteristic of that of the overall developer mixture. A firstphotoelectric transducer is positioned to be irradiated by the pulsedradiation reflected from the developer mixture and provides an outputsignal representative of the intensity of the reflected radiation.

A second photoelectric transducer, positioned to receive directly theradiation from the source, provides an output signal representative ofthe intensity of the emitted radiation as modulated by the surroundingenvironment. In one embodiment, the outputs of both photoelectrictransducers, after being passed through amplifiers, high pass filtersand peak detectors serve as the inputs to a divider network, the outputof which is representative of the ratio of the transducer outputs andcan be used as a control signal to activate a toner replenishment devicewhen the amplitude of the control signal is outside a preset range.Alternatively, the second transducer and associated amplifier, filterand peak detector are used as a feedback loop that corrects andmaintains the intensity of the output of the radiation source at aconstant value. The outputs from the first transducer, amplifier, fllterand peak detector which are inversely proportional to the tonerconcentration in the developer mixture, assuming the toner particleshave a higher optical reflection density than the carrier particles, areused to activate the toner replenisher whenever the amplitude of suchsignals rises to a predetermined value.

The apparatus of the invention has been found to be particularly welladapted for use with conventional magnetic brush developing assemblies,although it is contemplated that such apparatus can be adapted for usewith other types of developing assemblies, such as the cascade type.

The objects and various advantages of the present invention can best beunderstood from the ensuing detailed description of preferredembodiments, reference being made to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a front elevation view of anautomatic electrographic copier wherein the present invention hasutility;

FIG. 2 is a plan view of a magnetic brush development station embodyingthe invention, including a block diagram of toner concentrationmonitoring apparatus according to a preferred embodiment;

FIG. 3 is a cross sectional view of the magnetic brush developingstation depicted in FIG. 2 taken along the section line 33;

FIG. 4 is a detailed electrical schematic of the circuitry depicted inFIG. 2; and

FIG. 5 is a block diagram of another embodiment of the tonerconcentration monitoring apparatus of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS To assist theunderstanding of the present invention, the operation of anelectrographic copying machine in which the invention may be used willbe briefly described. It is to be understood, however, that theapparatus of the present invention could be used with equal facility andadvantage in other copying machines and, therefore, that the followingdescription of apparatus related to but not forming part of theinvention is pro vided for illustrative purposes only.

Referring now to the drawings and in particular to FIG. 1, anelectrophotographic copying machine wherein the invention isparticularly useful is shown to comprise an endless photoconductive belt2, driven about rollers 3 and 4 along a predetermined path by motor 5which is operably coupled with one of the rollers. Disposed along thepath are the various electrophotographic stations which serve to form atoner image of the document being reproduced on the outer surface ofbelt 2. As belt 2 passes charging station 6, its outer surface receivesa uniform electrostatic charge from a corona source or the like. Uponbeing uniformly charged, the belt is advanced past an exposure station 7where it is imagewise exposed to actinic radiation in accordance withthe light and dark areas of the original document. Such imagewiseexposure serves to selectively dissipate the uniform charge on the beltto form an electrostatic latent image corresponding to the indi cia onthe original document. Development of the electrostatic image isaccomplished as belt 2 is advanced past development station 8. Thelatter generally comprises a reservoir for containing an electrographicdeveloper, and means for applying the developer to the electrostaticimage so as to render the image visible. By employing appropriatelycharged toner particles, it is possible to produce a positive ornegative toner image of the original document. In order to reuse thatportion of the photoconductive belt bearing the toner image, the tonerimage is transferred to a paper receiving sheet 11 on which the tonerimage can be permanently fused. Such a transfer is commonly effected bya paper feeding device 12 which feeds sheets of paper from a papersupply 13 to a transfer station 14 simultaneously with the passagetherepast of the toner image bearing belt 2. A shift register R servesto control the timing of the electrophotographic operations and tosynchronize the feeding of the paper receiving sheets with the movementof the photoconductive belt. The shift register R includes a rotatablesegmented and slotted cylinder 20 which is driven by suitable means,such as belt 22 ex tending from a pulley around roller 3 so thatmovement of the shift register is in direct response to movement of thephotoconductive belt 2.

Transfer station 14 commonly comprises means for electrostaticallycharging the paper receiving sheet so as to attract toner particles frombelt 2 thereto. After the toner image is transferred to the paperreceiving sheet, the sheet is peeled away from the belt as the latterpasses over small roller 4. The toner-bearing receiving sheet is thenattracted by endless mesh belt transport 24, traveling about rollers 25in a clockwise direction and at the same speed as belt 2, and isadvanced thereby past a fusing station 30 where the toner image ispermanentized. The paper receiving sheet, with its toner-bearing surfacefacing downward, is caused to adhere to transport by a source ofnegative pressure on the rear surface of the lower leg of the transport.After fusing, the receiving sheet is dropped in a receptacle 31.

Referring now to FIGS. 2 and 3, the developing station 8 of theelectrophotographic apparatus described hereinabove may, for instance,be of the magnetic brush variety, commonly comprising a trough 50 forcontaining an electrographic developer 51, and a pair of conventionalmagnetic development brushes 52 and 53 for applying the developer to theelectrostatic image-bearing surface of belt 2. Each brush generallyincludes a rotatably mounted aluminum cylinder 54 having at least onemagnetic pole piece 55, shown in FIG. 3, interiorly disposed in a fixedposition along the longitudinal axis thereof. As previously mentioned,electrographic developer 51 generally comprises a mixture of toner andcarrier particles which adhere to each other under the influence oftriboelectric forces. In magnetic brush development, the carrierparticles are fabricated from a magnetically attractable material so asto be attracted to the surface of cylinder 54 by the magnetic fieldproduced by the internal pole pieces 55 to form chain-like arrays 57which resemble the bristles of a brush. A pair of rotating mixing augers58 and 59 serve to continously circulate the developer laterally throughtrough 50 and maintain the relative concentrations of the developercomponents substantially constant throughout the trough. Typically,drive means (not shown) are provided for rotating the developmentbrushes in a direction such that the movement of the brushes in thevicinity of contact with belt 2 is in a direction opposite the directionof travel of belt 2.

' As cylinders 54 rotate, developer 51 collects on the outer surfacesthereof under the influence of the magnetic field produced by theinternal pole pieces 55. As

the electrostatic image-bearing surface of belt 2 tangentially contactsthe developer-bearing cylinders,

toner is stripped from the carrier particles, due to the strongerelectrostatic forces, and selectively deposited on the belt surface toform toner images. As the cylinders continue to rotate, the partiallydenuded carrier particles used in forming the toner images are movedbeyond the influence of pole pieces 55 and fall back into the main bodyof the developer to be recoated with toner. Obviously, as toner imagesare continuously formed, the concentration of toner in the developmentmix gradually diminishes, the ultimate result being toner images ofgradually decreasing density.

Turning now to a more specific description of the subject matter of thepresent invention there is provided improved apparatus to continuouslymonitor the concentration of toner in the electrographic development mixand provide a signal whereby a conventional toner replenisher isactivated to maintain the concentration of toner at or above the levelrequired for high quality copies. Since the carrier and toner componentsof the development mixture commonly possess divergent reflectancecharacteristics, toner having a lower reflectivity than the carriercomponent, the reflectance of the development mix is an accurate measureof the relative concentrations of its two major components. The higherthe concentration of the carrier particles, the higher the reflectanceof the development mix. Stated otherwise, the developer reflectance isinversely proportional to the toner concentration.

Referring now to FlGS. 2 5 illustrating one embodiment of the invention,monitoring of the reflectance of the development mix 51 is accomplishedby directing the output of radiant energy source 60, preferably a lightemitting diode, toward a portion of the developer where theconcentration is representative of the average toner concentrationthroughout the development mix. In a magnetic brush development station,the toner concentration on the downstream development brush (i.e., brush53) between the points where the brush surface energes from thedevelopment mix and first contacts the surface of belt 2 is usuallycharacteristic of the average toner concentration. Between such points,the surface of the development brush comprises fresh developer which hasnot yet been subjected to the localized depletion of toner which resultsfrom development of the electrostatic image. Alternatively, tonerconcentration can be monitored on the upstream development brush (i.e.,brush 52) between the points where the brush surface loses contact withthe surface of belt 2 and reenters the development mixture. To

agitate the developer carried by the rush before being subjected to thetoner monitoring operation.

The use of a light-emitting diode as the source of radiation from whichdeveloper reflectance is determined offers the advantages overconventional sources of long-term stability, collimated output, fastresponse time, compactness and physical durability. Because of its fastresponse time, the light-emitting diode can be electronically pulsed atelectronic speeds (i.e., faster than several hundred Hertz), therebyoffering a means for readily discriminating against electricalinterference, commonly called noise, and normal environmental lightingwhich might illuminate the receiving element.

The periodic energization of the light-emitting diode is accomplished bymeans of a conventional pulse generator 67, comprising for example aunijunction oscillator and pulse shaping multivibrator (not shown),operating at' 1,000 Hertz.

'A lens 62 serves to concentrate the energy reflected from the developermixture on the brush surface onto the radiation sensitive surface of aphotoelectric transducer 61, preferably a silicon photodiode operatingin the zero-bias photovoltaic mode. The use of a photodiode as areceiving element offers the advantages over conventionalphotoconductive and photovoltaic cells of temperature stability andlinearity of operation, particularly when used in the photovoltaiczero-bias mode.

When a silicon photodiode is used as the receiving element, a galliumarsenide light-emitting diode is particularly preferred since thewavelength at which thediodes output peaks is in the near infraredspectral region which closely matches the peak spectral response of asilicon photodiode.

The special region wherein developer reflectance is monitored is chosenwith availabity and cost requirement of commercial light-emitting diodesand silicon photodetectors in mind; however, the most importantrequirement is that of choosing the wavelength at which the reflectancedisparity between toner and carrier is maximum, thereby producing thegreatest system sensitivity possible. Since the reflectance disparitybetween toner and carrier particles was found to be substantiallyconstant throughout the visible and near infrared spectral regions,efforts were restricted to matching the spectral output of thelight-emitting diode with the spectral response of the photodetector,and also to minimizing the sensitivity of the reflectance monitoringapparatus to normal environmental lighting. To maximize the use ofavailable energy, the lightemitting diode 60 and photodiode 61 arepreferably arranged side by side along and below the edge of belt 2, asshown in FIG. 3.

The a.c. output of the reflectance monitoring photodiode 61 is fed toamplifiers 80 and 82 and then to a high pass filter 91 comprisingcapacitor C4, resistor R18 and diode D8. The filter 91 filters thecomponents of the signals generated by the photodiode 61 which arerepresentative of the electrical interference of the circuitry and thedetected level of ambient light. The output of filter 91 is fed to apeak detector 63 which converts its input to a d.c. signal having anamplitude directly proportional to the reflectance of the developermixture 51 and, hence, inversely proportional to the instantaneous tonerconcentration. Peak detector 63 also functions to prevent isolated inputsignals from activating a conventional toner replenisher 64 such asdisclosed in US. Pat. No. 3,409,901. Consequently, the detector 63activates the toner replenisher 64 only when the amplitude of apredetermined number of input signals is outside a preset range.

Since the atmospheric environment wherein the re flectant monitoringapparatus of the invention is employed is normally contaminated by tonerparticles circulating in the air in the vicinity of the developmentstation 8, there is a tendency for toner to gradually accumulate on theexternal surfaces of the various elements of the monitoring apparatus,including the lightemitting diode. Such accumulation would, if notcompensated for, gradually reduce the output from diode 60, therebygradually reducing the energy received from photodiode 61. To stabilizethe output of the light-emitting 60, an electro-optical feedback loop isprovided which serves to gradually adjust the peak-topeak amplitude ofthe pulsed output of generator 67 as the modulated radiant output of thelight-emitting diode gradually changes. Such a feedback loop obviatesthe necessity of a periodic cleaning operation to maintain the optics ofthe monitoring apparatus in the same condition as existed at the time ofcallibration. The feedback looping includes a second photodiode 68disposed to receive modulated energy directly from light-emitting diode60, an a.c. amplifier 81, a high pass filter 92, a peak detector 69 anda d.c. amplifier 70. Like reflectance monitoring photodiode 61, thefeedback photodiode 68 is preferably operated in the zerobiasphotovoltaic mode. Similarly, filter 92 serves to filter the componentsof the signals generated by the photodiode 68 representative ofelectrical interference and ambient light. Peak detector 69 serves toconvert the pulsed a.c. output from the feedback photodiode 68 to a d.c.signal having an amplitude proportional to the peak-to-peak amplitude ofsuch output and to prevent spurious signals from effecting theadjustment of the output of light emitting diode 60. The output of thepeak detector 69 is amplified by the d.c. amplifier 70 and is fed backto pulse generator 67 to control the peak-to-peak amplitude of thepulses provided thereby.

By operating the photodiodes 61 and 68 in the photo voltaic, zero-biasmode, the dark current is exactly zero and the response is perfectlylinear. Since the dark current is zero, the photodiodes are practicallyinsensitive to the temperature variations normally associated with theinterior of the electrographic copying machine. Thus,temperature-compensation is unnecessary in the toner monitoringapparatus of the invention, unlike the apparatus disclosed in US. Pat.No. 3,399,652 wherein a thermistor is employed to stabilize systemsensitivity. However, operation at zero-bias necessitates the use of ahigh gain, amplifier, such as a conventional operational amplifier.Thus, the outputs of photodiodes 61 and 68 are applied to the inputs ofoperational amplifers and 81, respectively, to amplify their respectivephotocurrent outputs before being peak detected.

In FIG. 4 there is provided an electrical schematic which illustratespreferred circuits for maintaining the output of the light-emittingdiode 60 substantially constant and for deriving an electrical signalfrom the output of photodiode 61 by which the toner replenishingapparatus can be activated. As mentioned above, stabilization of theoutput of the light-emitting diode 60 is accomplished via anelectro-optical feedback which includes silicon photodiode 68 positionedto continuously sample the radiant output of light-emitting diode 60 asmodulated by the airborne toner particle cloud surrounding thedevelopment brushes 52 and 53. Operation of the photodiode 68 in thezero-bias photovoltaic mode is made possible through the use ofoperational amplifier 81 which functions as a currenttovoltagetransducer. The gain of amplifier 81 is determined by feedback resistorR1. The output of photodiode 68 constitutes a series of current pulses,the frequency and amplitude of which are proportional to the modulationfrequency and the environmentally modified irradiation intensity of thelight-emitting diode 60, respectively. Operation amplifier 81 serves toamplify and convert such current pulses to voltage pulses ofproportional amplitude. The a.c. output of amplifier 81 is coupled' tohigh pass filter 92 comprising capacitor C1 and diode D2. High passfilter 92 functions to filter the d.c. components of the signalsgenerated by photodiode 68 representative of undesired electricalinterference as well as low frequency a.c. signals representative of thelevel of ambient light detected by the photodiode. The output of highpass filter 92 is fed to peak detecting circuit 69 which comprises diodeD1, resistors R1 and R3, and capacitor C2. The d.c. output of the peakdetector 69, being directly proportional to the radiant output of thelight-emitting diode 60, is subsequently applied to the non-invertinginput of .the high gain differential d.c. amplifier 70. A controlvoltage derived from resistors R4, R5 and R6 is supplied to theinverting output of amplifier 70. The control voltage serves to balanceout the peak detector signal voltage so that the high-gain amplifier canbe utilized. Variable resistor R6 serves to provide an external controlon the operating point of the light-emitting diode 60. The values ofresistors R4 and R7 determine the gain of amplifier 70. Resistors R8 andR9 and diode D3 serve to prevent amplifier 70 from saturating on initialstartup. The output of amplifier 70 is a very sensitive indicator of theoutput of the light-emitting diode 60. That is to say, when the outputof the light-emitting diode 60 changes slightly, the output of amplifier70 will reflect this change with an amplification equal to its gain. Theoutput of amplifier 70 is fed back and combined with the input frompulse generator 67. Diodes D4 and D comprise a summing network tocombine the feedback voltage and the pulse generator input. Thissummation serves to adjust the pulse amplitude that appears at the baseof transistor Q1, which in turn drives the light emitting diode 60.Resistor R10 is a load resistor for amplifier 70 and the pulsegenerator. Transistor Q1 amplifies and inverts its pulse train inputwhich is sub sequently capacitively coupled through capacitor C3 to theDarlington current amplifier Q2 and Q3. Resistors R11 and R12 determinethe bias level and amplification factor of transistor Q1. Diode D6provides a discharge path for capacitor C3 through transistor Q1.Resistors R13 and R22 act to control the amplitude of the current pulsessupplied to light-emitting diode 60.

Also shown in FIG. 4 is circuitry for processing the output of thereflectance-monitoring photodiode 61. Operational amplifier 80 serves asa current-to-voltage transducer for converting the signal current fromthe photodiode 61 to a signal voltage. The signal voltage is amplifiedby voltage amplifier 82. Resistors R14, R15 and R16 are used to set thegain of amplifiers 80 and 82. Resistor R17 is used to balance the inputbias current for amplifier 82. The output of amplifier 82 is thencoupled to high pass filter 91 comprising capacitor C4 and diode D8.High pass filter 91 filters the dc. components of the signals generatedby photodiode 61 representative of undesired electrical interference aswell as low frequency a.c. signals representative of the level ofambient light detected by the photodiode. The output of filter 91 is fedto the peak detector 63 comprising resistors R18 and R19, diode D7, andcapacitor C5 which functions to convert its pulsed a.c. input to a do.output and to prevent isolated peak signals from activating the tonerreplenisher 64. Transistor Q4 and associated biasing resistors R20 andR21 are used as a buffer stage to obtain impedance matching to the tonerreplenishing circuitry. The output of transistor Q4 is proportional tothe instant radiation on the photodiode 61, which in turn isproportional to the reflectivity of the electrographic developer. Whenthe output signal from transistor Q4 exceeds a predetermined level,thereby indicating that the toner concentration has dropped below adesired level, the toner replenisher is activated.

Referring now to FIG. 5, circuitry is diagrammatically illustrated forcontinuously providing an electrical signal having an amplitudeproportional to the ratio of the outputs of photodiodes 61 and 68. Asshown, the output of the light-emitting diode 60 is used to directlyilluminate the photodiode 68 through the airborne toner cloudsurrounding the development brushes 52 and 53. The ac. output ofphotodiode 68 is amplified by operational amplifier 81 and filtered byhigh pass filter 92. The output of filter 92 is then peak detected bypeak detector 69, and the dc. output of the peak detector is fed as areference signal to one of the inputs of a conventional divider network86. The ac. output of the reflectance-monitoring photodiode 68, uponbeing amplified by amplifiers 80 and 82, filtered by high pass filter 91and peak detected by peak detector 63 serves as the other input to thedivider network. Such circuitry obviates the necessity for an opticalfeedback loop to maintain the output of the light-emitting didodesubstantially constant. It achieves long-term stability by alwaysnormalizing the output of photodiode 61 to the radiant output of thelight-emitting diode as modified by the toner cloud in the surroundingatmospheric environment. By this arrangement, any changes in the outputof the light-emitting diode 60 and any changes 5 in the density of thetoner cloud or in the accumulation of toner particles on the externalsurfaces of the optical components of the system will be automaticallyeliminated through the division provided by the divider network. Sincethis system is completely symmetric, its stability depends on the ratioof the errors in the two photodiodecircuits and on the stability of thedivider network. In this mode of operation, any errors in the twodetector circuits tend to cancel each other and the system stability hasbeen found to depend primarily on vider network is the Philbrick-NexusModel 4452 divider module. This device is hermetically sealed and has atemperature coefficient of only 3mvl C for a 25 C shift in temperature,there is less than 1 percent change in a 10 volt output signal.Alternatively, the divider network may comprise a digital computer inwhich case, of course, the detected amplified and filtered signals ofthe photodiodes are converted to a digital format prior to processing bythe computer. The computer performs the required calculation and outputsa signal to drive the toner replenisher 64.

The reflectance monitoring systems disclosed herein have been found tobe extremely sensitive to minute changes in the reflectivity of themixture being monitored, and exceptionally stable over extended periodsof operation. For these reasons, they have been found useful inmaintaining the relative proportions of mixed materials substantiallyconstant over extended periods even when the reflectance disparity ofsuch materials is very small. For instance, a change in tonerconcentration of from 0 to 7 percent (7 percent toner concentrationproducing total saturation or coverage of the carrier particles withtoner) results in a 25 percent change in the reflectivity of thedeveloper mixture. Since, for many developers, it is desirable tomaintain the toner concentration within the range of 4.75 to 5.25percent, it may be appreciated that the system must be sufficientlysensitive to detect changes in the reflectivity of the developer of theorder of 1 percent. By taking into account the changes in the density ofthe atmospheric environment in which the monitoring apparatus isemployed as well as the gradual accumulation of mixed materials on theexternal surfaces of such apparatus, the reflecting monitoring systemshave also been found to possess the stability required for maintainingthe toner concentration within the required range for several months ofcontinued use.

The invention has been described in detail with reference to preferredembodiments thereof but it will be understood that variations andmodifications can be effected within the spirit and scope of theinvention.

1 claim:

that of the divider network. A particularly useful di-' 1. ln anelectrographic machine having a development mechanism which brings adeveloper mixture of carrier particles and toner particles havingdivergent reflectance characteristics into contact with a latentelectrostatic image, the relative motion between the developer mixtureand the latent image creating a cloud of airborne toner particlessurrounding the development mechanism and an apparatus for regulatingthe relative proportion of the carrier and toner particles in thedeveloper mixture, the regulating apparatus including a source ofradiant energy directed toward the mixture, a first photo-electrictransducer illuminated by the radiation reflected from the developermixture, a second photoelectric transducer illuminated directly by theradiant energy source, and means responsive to the transducer outputsfor producing a control signal representative of the proportion of tonerand carrier particles in the mixture, the improvement wherein the secondphotoelectric transducer is positioned so as to receive radiation fromthe radiant energy source which passes through the airborne toner cloudand to provide an output signal representative of the intensity of theradiation produced by the radiant energy source as modulated by suchtoner cloud.

2. Apparatus for monitoring the proportion of a mixture of carrier andtoner particles having divergent reflectance characteristics, theapparatus being used in an environment including a circulating airbornecloud of such particles, said apparatus comprising:

a. a radiation source for projecting radiant energy upon a portion ofthe development mixture;

b. means for periodically energizing said source to produce pulses ofradiation at a selected frequency;

c. first photosensitive means positioned to receive pulses of radiationreflected from the development mixture, said first photosensitive meansbeing adapted to provide an output signal representative of thereflectivity of the mixture as modulated by the particle cloud;

d. second photosensitive means positioned to receive pulses of radiationfrom said source which pass through the particle cloud, said secondphotosensitive means being adapted to provide an output signalrepresentative of the intensity of the radiation produced by said sourceas modulated by the particle cloud;

e. first and second filters coupled with the output signals from saidfirst and second photosensitive means for filtering signals ofnon-selected frequencies; and

f. means coupled to said first and second filters and responsive to theoutput signals therefrom for producing a signal representative of theproportion of toner particles in the mixture.

3. The invention as defined in claim 2 wherein said source of radiantenergy comprises a light emitting diode.

4. The invention as defined in claim 2 further comprising a pair of peakdetectors operatively connected to said first and second photosensitivemeans and adapted to provide d.c. signals proportional to the peak topeak outputs of said first and second photosensitive means.

5. Apparatus for monitoring the proportion of mixed materials havingdivergent reflectance characteristics, said apparatus comprising:

a. a container for the mixed materials;

b. a luminous diode capable of emitting infrared energy when energized;

c. means for periodically energizing said diode to produce pulses ofinfrared energy at a selected frequency;

d. means for directing said pulses of infrared energy toward the mixedmaterials;

e. a first photoelectric transducer responsive to said infrared energypositioned to receive said pulses of infrared energy after reflection bythe mixed materials, said first photoelectric transducer being adaptedto produce an output signal having an informational contentrepresentative of the reflectivity of the mixture as modulated by theinterior environment of said container;

a second photoelectric transducer responsive to said infrared energypositioned to receive said pulses of infrared energy directly from saiddiode, said second photoelectric transducer being adapted to produce anoutput signal having an informational content representative of theintensity of radiation emitted from said diode as modulated by theinterior environment of said container;

g. a pair of high pass filters coupled to the outputs of said first andsecond photoelectric transducers and adapted to pass signals only ofsaid selected frequency;

h. a pair of peak detectors coupled to the outputs of said first andsecond photoelectric transducers through said filters, said peakdetectors being adapted to provide d.c. signals proportional to the peakto peak outputs from said photoelectric transducers; and

i. means coupled to the outputs of said peak detectors and adapted toprovide a signal proportional to the ratio of said peak detectoroutputs, said ratio being proportional to the proportion of the mixedmaterials;

6. The invention as defined in claim 5 wherein said means coupled to theoutputs of said peak detectors comprises a divider network.

7. Apparatus for monitoring the proportion of a development mixture ofcarrier and toner particles having divergent reflectancecharacteristics, the apparatus being used in an environment including acirculating airborne cloud of such particles, said apparatus comprising:

a. a radiation source for projecting radiant energy upon a portion ofthe development mixture;

b. means for periodically energizing said source to produce pulses ofradiation at a selected frequency;

c. first photosensitive means positioned to receive pulses of radiationreflected from the development mixture, said first photosensitive meansbeing adapted to provide an output signal representative of thereflectivity of the mixture as modulated by the particle cloud;

d. a first high pass filter coupled to said first photosensitive meansfor filtering output signals of nonselected frequencies;

e. means coupled to said first high pass filter for regulating theproportion of the particles of the development mixture in response to apredetermined change in the amplitude of the output signals from saidfirst high pass filters;

f. second photosensitive means positioned to receive pulses of radiationfrom said source which pass through the particle cloud, said secondphotosensitive means being adapted to provide an output signalrepresentative of the intensity of the radiation produced by said sourceas modulated by the particle cloud; and

g. a second high pass filter coupled to said second photosensitive meansfor filtering output signals of non-selected frequencies, the outputsignals of said second filter being coupled to said radiation source tomaintain the intensity of the radiation output produced by said sourceat a constant value.

8. The invention as defined in claim 7 further comprising first andsecond peak detectors coupled to said first and second filters,respectively, and adapted to provide d.c. signals proportional to theoutputs of said first and second filters.

9. For use in an electrographic machine in which a surface carrying anelectrostatic charge pattern is contacted by the bristles of anelectrographic magnetic development brush, the bristles comprising anelectrographic development mixture of carrier particles and tonerparticles having divergent reflectance characteristics, the contact ofthe charge pattern by the brush bristles producing an airborne tonerparticle cloud surrounding the development brush, an apparatus forregulating the proportion of such particles in the mixture, saidapparatus comprising:

a. a source of radiant energy; b. means for periodically energizing saidsource to produce pulses of radiation at a selected frequency;

c. means for directing said pulses of radiation emanating from saidsource toward the development brush to irradiate a portion of thedevelopment mixture thereon;

d. a first photoelectric transducer positioned to receive said pulses ofradiation after reflection by the development mixture, said firstphotoelectric transducer being adapted to provide a first output signalrepresentative of the reflectivity of the mixture on the developmentbrush as modulated by the surrounding toner particle cloud;

e. a second photoelectric transducer positioned to receive said pulsesof radiation from said source which pass through the toner particlecloud, said second photoelectric transducer being adapted to provided asecond output signal representative of the intensity of radiationemanating from said source as modulated by the surrounding toner particle cloud;

f. first and second high-pass filters connected to the outputs of saidfirst and second photoelectric transducers, respectively, for filteringsignals of nonselected frequencies; and

g. means responsive to said filtered first and second output signals andadapted to produce a third output signal representative of theproportion of toner particles in the mixture.

10. The invention as defined in claim 9 further comprising convertermeans coupled to said first and second filters and adapted to provided.c. signals proportional to the outputs of said first and secondfilters.

11. The invention as defined in claim 9 further comprising meansextending into the brush bristles for agitating the development mixturebefore the mixture is subjected to said pulses of radiation emanatingfrom said source.

i233? v UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No.3, 3 A Dated August 97 Inventor(s) BenWOOd 6t e11 It is certified thaterror appears in the above-identified patent and that said LettersPatent are hereby corrected as shown below:

1, line 17, delete "Pat.".

I 2, line 35, "surface" should read ii surfaces Column 6, line 36,"rush" should read -brush-.

Column 0, line 8, 'didode" should read -diode--.

Signed and sealed this- 11th day of February 1975.

(SEAL) I Att St:

e C. MARSHALL DANN RUTH C. MASON Commissioner of Patents AttestingOfficer and Trademarks 73 UNITED STATES PATENT OFFICE CERTIFICATE OFCORRECTION Patent No. 3, 3 A D d August 20, 197

Inventor(s) BenWOOd et al It is certified that error appears in theabove-identified patent and that said Letters Patent are herebycorrected as shown below:

I- 1| H I q Column 1, llne l7, delete Pat. Column 2, line 35, "surface"should read -I SurfaCeS Column 6, line 36, "rush" should read --brush-.Column 10, line 8, "didode" should read -diode--.

Signed and sealed this 11th day of February 1975-.

(SEAL) I A t t:

t es C. MARSHALL DANN RUTH C. MASON Commissioner of Patents AttestingOfficer and Trademarks

1. In an electrographic machine having a development mechanism whichbrings a developer mixture of carrier particles and toner particleshaving divergent reflectance characteristics into contact with a latentelectrostatic image, the relative motion between the developer mixtureand the latent image creating a cloud of airborne toner particlessurrounding the development mechanism and an apparatus for regulatingthe relative proportion of the carrier and toner particles in thedeveloper mixture, the regulating apparatus including a source ofradiant energy directed toward the mixture, a first photo-electrictransducer illuminated by the radiation reflected from the developermixture, a second photoelectric transducer illuminated directly by theradiant energy source, and means responsive to the transducer outputsfor producing a control signal representative of the proportion of tonerand carrier particles in the mixture, the improvement wherein the secondphotoelectric transducer is positioned so as to receive radiation fromthe radiant energy source which passes through the airborne toner cloudand to provide an output signal representative of the intensity of theradiation produced by the radiant energy source as modulated by suchtoner cloud.
 2. Apparatus for monitoring the proportion of a mixture ofcarrier and toner particles having divergent reflectancecharacteristics, the apparatus being used in an environment including acirculating airborne cloud of such particles, said apparatus comprising:a. a radiation source for projecting radiant energy upon a portion ofthe development mixture; b. means for periodically energizing saidsource to produce pulses of radiation at a selected frequency; c. firstphotosensitive means positioned to receive pulses of radiation reflectedfrom the development mixture, said first photosensitive means beingadapted to provide an output signal representative of the reflectivityof the mixture as modulated by the particle cloud; d. secondphotosensitive means positioned to receive pulses of radiation from saidsource which pass through the particle cloud, said second photosensitivemeans being adapted to provide an output signal representative of theintensity of the radiation produced by said source as modulated by theparticle cloud; e. first and second filters coupled with the outputsignals from said first and second photosensitive means for filteringsignals of non-selected frequencies; and f. means coupled to said firstand second filters and responsive to the output signals therefrom forproducing a signal representative of the proportion of toner particlesin the mixture.
 3. The invention as defined in claim 2 wherein saidsource of radiant energy comprises a light emitting diode.
 4. Theinvention as defined in claim 2 further comprising a pair of peakdetectors operatively connected to said first and secoNd photosensitivemeans and adapted to provide d.c. signals proportional to the peak topeak outputs of said first and second photosensitive means.
 5. Apparatusfor monitoring the proportion of mixed materials having divergentreflectance characteristics, said apparatus comprising: a. a containerfor the mixed materials; b. a luminous diode capable of emittinginfrared energy when energized; c. means for periodically energizingsaid diode to produce pulses of infrared energy at a selected frequency;d. means for directing said pulses of infrared energy toward the mixedmaterials; e. a first photoelectric transducer responsive to saidinfrared energy positioned to receive said pulses of infrared energyafter reflection by the mixed materials, said first photoelectrictransducer being adapted to produce an output signal having aninformational content representative of the reflectivity of the mixtureas modulated by the interior environment of said container; f. a secondphotoelectric transducer responsive to said infrared energy positionedto receive said pulses of infrared energy directly from said diode, saidsecond photoelectric transducer being adapted to produce an outputsignal having an informational content representative of the intensityof radiation emitted from said diode as modulated by the interiorenvironment of said container; g. a pair of high pass filters coupled tothe outputs of said first and second photoelectric transducers andadapted to pass signals only of said selected frequency; h. a pair ofpeak detectors coupled to the outputs of said first and secondphotoelectric transducers through said filters, said peak detectorsbeing adapted to provide d.c. signals proportional to the peak to peakoutputs from said photoelectric transducers; and i. means coupled to theoutputs of said peak detectors and adapted to provide a signalproportional to the ratio of said peak detector outputs, said ratiobeing proportional to the proportion of the mixed materials;
 6. Theinvention as defined in claim 5 wherein said means coupled to theoutputs of said peak detectors comprises a divider network.
 7. Apparatusfor monitoring the proportion of a development mixture of carrier andtoner particles having divergent reflectance characteristics, theapparatus being used in an environment including a circulating airbornecloud of such particles, said apparatus comprising: a. a radiationsource for projecting radiant energy upon a portion of the developmentmixture; b. means for periodically energizing said source to producepulses of radiation at a selected frequency; c. first photosensitivemeans positioned to receive pulses of radiation reflected from thedevelopment mixture, said first photosensitive means being adapted toprovide an output signal representative of the reflectivity of themixture as modulated by the particle cloud; d. a first high pass filtercoupled to said first photosensitive means for filtering output signalsof non-selected frequencies; e. means coupled to said first high passfilter for regulating the proportion of the particles of the developmentmixture in response to a predetermined change in the amplitude of theoutput signals from said first high pass filters; f. secondphotosensitive means positioned to receive pulses of radiation from saidsource which pass through the particle cloud, said second photosensitivemeans being adapted to provide an output signal representative of theintensity of the radiation produced by said source as modulated by theparticle cloud; and g. a second high pass filter coupled to said secondphotosensitive means for filtering output signals of non-selectedfrequencies, the output signals of said second filter being coupled tosaid radiation source to maintain the intensity of the radiation outputproduced by said source at a constant value.
 8. The invention as definedin claim 7 further comprising first and second peak deteCtors coupled tosaid first and second filters, respectively, and adapted to provide d.c.signals proportional to the outputs of said first and second filters. 9.For use in an electrographic machine in which a surface carrying anelectrostatic charge pattern is contacted by the bristles of anelectrographic magnetic development brush, the bristles comprising anelectrographic development mixture of carrier particles and tonerparticles having divergent reflectance characteristics, the contact ofthe charge pattern by the brush bristles producing an airborne tonerparticle cloud surrounding the development brush, an apparatus forregulating the proportion of such particles in the mixture, saidapparatus comprising: a. a source of radiant energy; b. means forperiodically energizing said source to produce pulses of radiation at aselected frequency; c. means for directing said pulses of radiationemanating from said source toward the development brush to irradiate aportion of the development mixture thereon; d. a first photoelectrictransducer positioned to receive said pulses of radiation afterreflection by the development mixture, said first photoelectrictransducer being adapted to provide a first output signal representativeof the reflectivity of the mixture on the development brush as modulatedby the surrounding toner particle cloud; e. a second photoelectrictransducer positioned to receive said pulses of radiation from saidsource which pass through the toner particle cloud, said secondphotoelectric transducer being adapted to provided a second outputsignal representative of the intensity of radiation emanating from saidsource as modulated by the surrounding toner particle cloud; f. firstand second high-pass filters connected to the outputs of said first andsecond photoelectric transducers, respectively, for filtering signals ofnon-selected frequencies; and g. means responsive to said filtered firstand second output signals and adapted to produce a third output signalrepresentative of the proportion of toner particles in the mixture. 10.The invention as defined in claim 9 further comprising converter meanscoupled to said first and second filters and adapted to provide d.c.signals proportional to the outputs of said first and second filters.11. The invention as defined in claim 9 further comprising meansextending into the brush bristles for agitating the development mixturebefore the mixture is subjected to said pulses of radiation emanatingfrom said source.